Snapping in 3D indicates which point, edge, surface or center is going to be picked.
The available snap points are dependent on which tool (Measurement, Markup or Clip Plane) you are using.
Snapping preferences can be defined per tool (Measurement, Markup or Clip Plane). When the tool is activated a toolbar will display at the bottom of the screen.
The default setting is Auto, which will snap to the closest edge, point, face or center that your cursor is near.
Snap to center enabled. The selected tool will only snap to centers.
You are not limited to only selecting 1 snap preference. When 2 preferences are selected, the application will snap to the closest preference (edge, point, face or center) that your cursor is near.